1. Field of the Invention
The present invention relates to a magnetic resonance imaging apparatus and a magnetic resonance imaging method which generate gradient magnetic fields on an imaging area formed in a static magnetic field, resonates nuclear spins in the object set in the imaging area magnetically by transmitting radio frequency signals and reconstructs an image of the object by using nuclear magnetic resonance signals generated due to an excitation, and more particularly, to a magnetic resonance imaging apparatus and a magnetic resonance imaging method in which influence of motion of the object on an image is reduced by correction.
2. Description of the Related Art
An MRI (Magnetic Resonance Imaging) apparatus is an apparatus which generates gradient magnetic fields by gradient coils on an imaging area of an object set in a cylindrical static field magnet for producing a static magnetic field, resonates nuclear spins in the object magnetically by transmitting RF (Radio Frequency) signals from an RF coil and reconstructs an image of the object by using NMR (Nuclear Magnetic Resonance) signals generated due to an excitation.
Imaging of the heart under a magnetic resonance imaging method with use of the magnetic resonance imaging apparatus has been further developed in recent years. Typical applications of the imaging of the heart include high resolution imaging for the blood vessel figure of the coronary artery. In the high resolution imaging for the coronary artery, an influence of a breathing motion on an image needs to be reduced as much as possible.
One of measurements for suppressing the influence of the breathing motion is breath-holding imaging in which the imaging is performed when the breath is held. However, in the breath-holding imaging, the imaging is only performed during a breath-holding period and there is a limitation on the resolution. In addition, there is a fear in the degree of stability of the breath-holding.
Another method of suppressing the influence of the breathing motion relies on a technique of using synchronous imaging as well under a free breath condition. A synchronous signal used in the synchronous imaging can be obtained by an expansion and contraction sensor or a pressure sensor arranged around the abdominal part of an object. However, there is a problem in that this synchronous imaging method with use of the synchronous signal obtained by the expansion and contraction sensor or the pressure sensor has insufficient accuracy.
In view of the above, another method instead of the synchronous imaging method with use of the expansion and contraction sensor or the pressure sensor, there are proposed a synchronous imaging method in which the position of the diaphragm detected on the basis of NMR signals from the diaphragm is used as a synchronous signal (for example, Liu et al., Magnetic Resonance In Medicine, 30, pages 507-511, (1993))and an imaging method for reflecting positional information of a moving imaging area on a control for collecting the NMR signals for imaging to finely adjust an excited slice position.
FIG. 22 is a diagram explaining an area for acquiring NMR signals for detecting motion of a diaphragms in a conventional magnetic resonance imaging apparatus. FIG. 23 is a diagram showing a conventional pulse sequence defining an imaging condition to acquire data for detecting motion and that for imaging shown in FIG. 22.
In a synchronous imaging method using the position of the diaphragm as the synchronous signal, as shown in the solid line frame of FIG. 22, other than a data collection area for imaging including the heart, an area on the cylinder as shown in the dotted line frame including the diaphragm is set as the data collection area for the motion detection.
Then, the imaging is executed in accordance with the pulse sequence shown in FIG. 23. In the general pulse sequence, a pre-pulse such as a fat suppression pulse is used in combination in many times, and prior to the sequence for imaging, a sequence for applying the pre-pulse is set. Then, prior to the sequence for the pre-pulse, a sequence for the motion detection is set.
In addition, a sequence for applying dummy shots (which is also referred to as stabilization shots) for acquiring data necessary for post-processing on data or information which is required in collecting data for imaging is set at the start of the data collection for imaging. Normally, the slice direction of the dummy shot is set to an axial cross-section for the purpose of realizing the stabilization of spinning similarly to the data collection for imaging.
Then, on the basis of the sequence for the motion detection, the data collection area for the motion detection including the diaphragm is excited in a particular condition different from the exciting method for the imaging area. Furthermore, the data for the motion detection is acquired from the collection area including the diaphragm and a signal called navigator is generated. Next, the position of the diaphragm is detected from the navigator signal, and a control method for hardware at the time of imaging is determined and adoption judgment as to the data collection for imaging is conducted in accordance with the amount of change in the position of the diaphragm. In addition, the amount of the breathing-related shift on the heart necessary for the motion correction on the data for imaging is calculated by multiplying the amount of shift of the diaphragm by a given ratio.
Such a navigator method in which the sequence for the motion detection which is different from the sequence for imaging is used for collecting the navigator signal to acquire the synchronous signal is applied to various technologies.
However, the navigator method for collecting the navigator signal under a condition different from the data collection condition for imaging has two major problems.
The first problem resides in that the amount of breath-related shift of the target area of the imaging (the heart) and the amount of shift of the area to be observed by the navigator signal (the diaphragm) have relevance but are not completely the same to each other. Therefore, the amount of shift of the heart is estimated from the amount of shift of the diaphragm, which is a cause of decreasing the accuracy. In addition, a ratio between the amount of breath-related shift of the heart and the amount of shift of the diaphragm varies between individuals and changes depending on the state of breathing even in the same object, and it is therefore difficult to obtain a stable image.
The second problem resides in that timing for detecting the signal from the diaphragm is largely different from timing for detecting the signal for imaging. It is necessary to continuously perform the application of the pre-pulse and the data collection for imaging to be executed after application of the pre-pulse. Therefore, the timing for collecting the navigator signal on the basis of the sequence for collecting the navigator signal (navigator sequence) disadvantageously needs to be separated in view of time from timing for collecting the data on the basis of the sequence for imaging. Then, the shift of the timing for collecting the navigator signal from the timing for the data collection for imaging becomes a cause of decreasing the accuracy in a case where a breath period of the object is relatively short.
On the other hand, as another method for detecting the motion of the diaphragm, a method of generating a navigator signal even during data collection for imaging by using a pulse sequence for imaging is proposed (for example, Ehman, Felmee, Radiology, 173, pages 255-263, (1989)).
This technique for generating the navigator signal during the data collection for imaging includes generating a plurality of echo signals in a spin echo sequence and utilizing one of the thus generated echo signals as the navigator signal. According to this technique, the motion information in the readout direction and the phase encode direction can be observed. Then, this technique has a merit of collecting the navigator signal and the signal for imaging substantially at the same timing as well as a merit of observing the motion of the same area as the imaging target, that is, the motion of the heart.
However, the conventional technology for generating the navigator signal during the data collection for imaging suffers a problem of difficulty in observing the motion at a practically sufficient accuracy. This accuracy deficient problem arises because the navigator signal is data obtained by projecting data from the object in a particular direction. That is, the navigator signal is data obtained by superposing data from the area which is not in motion in actuality onto data from the area which is in motion as being the projection data in the particular direction. Thus, the navigator signal is under influence of the area that is not in motion.
For example, in the thoraco-abdominal area, fats on the body surface, the chest wall, muscles of the back, and the like are not in motion. However, these unmoved areas are closer to the reception coil than moving areas such as the liver, and accordingly the signal intensity from the unmoved area is relatively larger than the signal intensity from the moving area. Therefore, the accuracy in detection of the motion is easily influenced by the unmoved area, and particularly in a case of imaging requiring a high accuracy such as high resolution imaging, the influence of the unmoved area on the motion detection accuracy becomes a problem.